The present invention relates to a photoconductive material. More particularly, it relates to a photoconductive material showing changes in electrical conductivity upon changes in temperature. The invention further relates to a switching material, which is one of the photoconductive materials. In particular the present invention relates to a novel switching material which can be used as a material for the manufacture of photoelectric switches capable of sensing light in the ultraviolet region or temperature switches capable of sensing very low temperatures not higher than 150 K.
Photoconductive materials that show an increased electrical conductivity upon light irradiation and thus facilitate electric current flow are used in light sensors, camera tube photoelectric surfaces, electronic drums and the like. Typical examples of such materials are semiconductors made of silicon, germanium or the like.
However, none of these known photoconductive materials has been found to be a material capable of markedly changing its photoconductivity upon temperature changes.
Therefore, in the case of temperature switches manufactured by using the conventional photoconductive materials, the signal/noise (S/N) ratio on the occasion of switching on or off is not very great since the change in photoelectric current is small. When temperature switches comprising a photoconductive material having such characteristics are used in electric circuit elements, operating errors may possibly be caused by external noises.
Photoconductivity is a characteristic observed with a very large number of materials, for example, nonmetal solid-form simple substances, and sulfides. Various switching materials in which such photoconductivity characteristics is utilized have been provided.
However, these prior art switching materials cannot serve in the ultraviolet region since they rely on their spectral characteristics in connection with visible or infrared light. The lowest temperature limit at which they still can serve is as high as about -30.degree. C. None of the prior art switching materials is known to be usable at such very low temperatures as 150 K. or below.
On the other hand, for semiconductors such as silicon or germanium, the temperature function of the electrical conductivity changes merely exponentially, without showing any abrupt conductivity change at a certain specific temperature. It is out of the question to apply such semiconductors to temperature switches for sensing such an extremely low temperature as 150 K.
In the case of composite materials made by depositing an oxide conductive thin film layer on a substrate made of silicon or the like known photoelectric material at an elevated temperature (e.g. 500.degree. C.), an insulator material is formed in the interface between the substrate and thin film, hence electric connection between the substrate and thin film is impossible.